This invention relates to systems for fabricating microcircuitry by causing electron beam images of circuit patterns formed by masks to be projected upon sensitized silicon substrates or wafers, and it relates particularly to a microfabrication system of the type in which the electron beam image is projected from an irradiated photocathode window that is selectively masked to define the pattern areas from which photoelectrons may be emitted to form an electron beam having the desired image configuration.
The photocathode projection method just described has an advantage over other types of electron image-forming techniques in that the mask is not required to be self-supporting; hence it may be formed exactly in accordance with the desired pattern without having to provide supporting connections to unexposed areas that are completely surrounded by exposed areas. Thus, it avoids the kind of image distortion known as the "stencil problem". As constructed heretofore, however, photocathode projection devices have been subject to certain disadvantages. For instance, if contaminants emitted by the resist-coated substrate reach the photocathode, they have an adverse effect upon the photocathode and shorten its life. In prior structures of this type there has been no feasible way to shield the photocathode from such contaminants.
Other disadvantages of prior photocathode projection devices have arisen from the fact that in such devices it is customary to have the wafer surface exposed to the electrostatic field which drives the electrons from the photocathode to the wafer. This makes it difficult to register the projected image with respect to alignment marks on the wafer, because any registration signal detector which is employed to sense the electrons that are backscattered from the alignment marks will have to be positioned within the electrostatic field, causing perturbations therein which tend to distort the image. Registration detectors utilizing X-rays have been proposed to overcome this problem, but they have not proved satisfactory in practice. Another disadvantage of having the wafer positioned in the electrostatic field is that any variations in the flatness of the wafer surface will cause perturbations of the electrostatic field, with consequent distortion of the reproduced image. Moreover, as the electron beam impinges areas of the wafer surface that correspond to the unmasked areas of the photocathode, some of the backscattered electrons produced by the beam will be driven by the electrostatic field back onto portions of the wafer surface which correspond to masked areas of the photocathode, thereby lessening the contrast between masked and unmasked areas and lowering the quality of the circuit pattern produced on the wafer, as well as making it difficult to detect alignment marks accurately.
Still another disadvantage of prior photocathode projection devices is the fact that they are inherently limited to a 1:1 reproduction ratio. Therefore, the circuit pattern on the photocathode must be of the same small size as the image thereof which is being microfabricated on the wafer, making it difficult and/or expensive to form a highly accurate pattern on the photocathode.